Which of the following is the correct relation regarding coupled coils?

• A. M > (L1L2)0.5
• B. M = (L1 − L2)/2
• C. M = (L1 + L2)/2
• D. M ≤ (L1L2)0.5

Answer: (D)

Mutual inductance between two coupled coils is given by the relation
M = K√L1L2
Where
L1L2 = Inductance of the coil
K = Coefficient of coupling
When K = 1 coils are said to be tightly coupled and if K is a fraction the coils are said to be loosely coupled.
Here
Kmax =1
Kmin =0
∴ m ≤ (L1 L2 )0.5